Field of the Invention
The present invention relates to a hydraulic system for a working machine such as a backhoe, the hydraulic system including a load sensing system.
Description of the Related Art
Japanese Unexamined Patent Application Publication No. 2014-1563 (Patent Document 1) and Japanese Examined Utility Model Registration Publication No. 2572936 (Patent Document 2) each disclose a hydraulic system for working machines as the hydraulic system for a working machine, the hydraulic system including a load sensing system.
The hydraulic system for working machines disclosed in Japanese Unexamined Patent Application Publication No. 2014-1563 (Patent Document 1) includes: a plurality of hydraulic actuators; a control valve; a variable displacement pump; a regulator; and a pilot pump. The control valve is configured to control the hydraulic actuators. The variable displacement pump is configured to supply a hydraulic operation fluid (a hydraulic operation oil) to the hydraulic actuators. The regulator is configured to control the variable displacement pump. The pilot pump is configured to discharge a hydraulic pilot fluid (a hydraulic pilot oil). The hydraulic system for working machines further includes a load sensing system. The load sensing system is configured to control a discharge pressure from the variable displacement pump with the regulator, thereby obtaining a constant differential pressure between a PPS (Pressure of Pump Sensing) signal pressure and a PLS (Pressure of Load sensing) signal pressure. The differential pressure is a subtraction between the PLS signal pressure and the PPS signal pressure, the PLS signal pressure being the maximum load pressure of load pressures in the hydraulic actuators, the PPS signal pressure being the discharge pressure from the variable displacement pump.
In addition, the hydraulic system is provided with a PLS signal tube for transferring the PLS signal pressure to the regulator. The PLS signal tube is disposed between the control valve and the regulator, extending from the control valve to the regulator. Moreover, the hydraulic system is provided with a throttle in the middle of the PLS signal tube for a stable operation.
The hydraulic system for working machines disclosed in Japanese Examined Utility Model Registration Publication No. 2572936 (Patent Document 2) includes a load sensing system constituted of: a variable displacement pump; a hydraulic actuator; an operation valve; a displacement control cylinder; a pilot cylinder; and a load sensing valve. The hydraulic actuator is driven by the variable displacement pump. The operation valve is provided on a tube disposed between the variable displacement pump and the hydraulic actuator. The displacement control cylinder controls a displacement of the variable displacement pump. The pilot cylinder has a spring at one end of the pilot cylinder, the spring being for setting a differential pressure. The pilot cylinder is moved to a position for supplying a control pressure to the displacement control cylinder to be connected to a portion on an upstream side of the operation valve, and the pilot cylinder is moved to a position for discharging the control pressure of the displacement control cylinder to be connected to a portion on a lower stream side of the operation valve. The hydraulic system is provided with a tube for connecting both of the pilot cylinders of the load sensing valve to each other, the tube including a switch valve for switching a state of the tube to an opened state and to a closed state.
The hydraulic system sets the switch valve to the opened state when a fluid temperature (an oil temperature) is low, the switch valve being configured to open and close the tube for connecting both of the pilot cylinders of the load sensing valve to each other. Then, the differential pressure between both of the pilot cylinders of the load sensing valve disappears, and thereby the load sensing valve is moved, by the spring for setting the differential pressure, to the position for discharging the control pressure of the displacement control cylinder. Accordingly, the hydraulic system controls the displacement of the variable displacement pump to be maximized to increase an amount of the hydraulic operation fluid relieved from a safety valve, the hydraulic operation fluid being of the hydraulic operation fluid discharged from the hydraulic pump, and thereby a loss of the pressure of the relieved hydraulic operation fluid rapidly increases the temperature of the hydraulic operation fluid.